This invention relates generally to comparator circuits and more particularly to comparator circuits having high gain to rapidly and accurately convert a differential input signal into a logical signal.
As is known in the art, comparator circuits have a wide range of applications where, for example, the voltage level of an input signal is compared with a reference voltage and a logical signal is produced at the output of the comparator circuit; the logic state of the logical signal being in accordance with whether the voltage level of the input signal is greater than, or less than, the level of the reference voltage.
As is also known in the art, some types of comparator circuits include an input stage coupled to an output stage through at least one intermediate stage and a level converter. One such comparator circuit is described in an article entitled "A Fast, Latching Comparator for 12-bit A/D Applications" by George Erdi, published in the IEEE Journal of Solid State Circuits, Volume SC-15, No. 6, December 1980. In such comparator circuit, the output stage includes a pair of common emitter transistors having base electrodes coupled to the output of a second gain stage and collector electrodes coupled to a pair of cascode, common base configured transistors. The collector electrode of one of the pair of cascode connected transistors is coupled to the base of an output transistor through an emitter follower transistor. The base electrode of the output transistor is connected to its grounded emitter electrode through a resistor which provides a passive discharge path for base charge when such output transistor is driving to a nonconducting condition. The parallel combination of the base-emitter resistance of the output transistor and the discharge resistor connected to the base of the output transistor reduces the gain provided by the emitter follower transistor to the output transistor. The collector electrode of the other one of the pair of cascode connected transistors is connected to a voltage source through a resistor and also to the base electrode of a pull-up transistor which has its emitter and collector electrodes connected between the voltage source and the collector of the output transistor. The resistor in the base electrode circuit of the pull-up transistor increases the time required to drive the output transistor to a conducting condition because of the relatively large time constant created by such resistor and the inherent capacitance at the base electrode of the pull-up transistor. Thus, while such comparator may be used in some applications, the relatively large resistor in the base electrode circuit of the pull-up transistor and the passive resistor in the base discharge path of the output transistor reduces the gain and conversion speed of the comparator circuit.
In another comparator circuit, described on page 2-27 of Advanced Micro Devices Linear and Interface Data Book, published by Advanced Micro Devices, Inc., 1979, the base electrode of the output transistor of the output stage is connected to the collector of one of a pair of differential common emitter transistors providing the input for the output stage. In such circuit, base charge for the output transistor is provided through a relatively large resistor in the collector circuit of one of the transistors in the pair of differential common emitter transistors thereby reducing the conversion speed of the circuit. Further, the high frequency gain provided by the differential common emitter transistors is limited because of the Miller capacitance effect between the base and collector electrodes of such transistors, thereby limiting the conversion speed of the circuit.